Spark ignition circuit



Aug. 26, 1969 R, K, FA|RLEY ETAL 3,463,963

SPARK IGNITION CIRCUIT Filed D66. 2B, 1957 SPARK IGNITION CIRCUIT Richard K. Fairley, Deerfield, and Reinhold Mueller,

Arlington Heights, Ill., assignors to Controls Company of America, Melrose Park, Ill., a corporation of Delaware Filed Dec. 28, 1967, Ser. No. 694,200 Int. Cl. Hb 41/14 U.S. Cl. 315-206 19 Claims ABSTRACT OF THE DISCLOSURE In the ignition circuit spark electrodes are connected in the secondary of an ignition transformer and, with respect to the electrical source, a capacitor and a silicon controlled rectifier (SCR) are connected in series with each other and the ignition transformer primary. The SCR is initially non-conductive interrupting the electrical circuit of the capacitor and primary and is rendered conductive to momentarily complete that circuit to provide a steep wave front pulse to the transformer primary. The operational state of the SCR is controlled by a voltage divider circuit which determines the voltage drop `across the anode and gate of the SCR. In one form the ignition circuit includes a thermistor connected in one branch of the voltage divider circuit so that the thermistor determines the relative voltage condition of the SCR anode and gate in accordance with a temperature condition to which the thermistor is exposed. In another alternative circuit this relative voltage condition of the SCR gate is determined by a voltage divider circuit which includes the primary of a voltage step-down transformer, the secondary of the transformer being connected to a low voltage thermostat exposed to a temperature condition which is to be the basis of control. The change in operational state of the thermostat is transmitted through the transformer coils and is reflected in a change in impedance of the transformer primary altering the voltage condition of the voltage divider circuit and the operational state of the SCR in accordance with the sensed temperature condition.

In a specific aspect of this disclosure, the ignition transformer is wound such that its secondary coil is arranged in a plurality of serially arranged coil portions, a continuation of the coil electrically connecting adjacent coil portions with the coil portions being otherwise electrically insulated one from the other.

BACKGROUND OF INVENTION Field of invention-This invention relates to spark ignition circuits.

Description of prior arb-Various forms of spark ignition circuits, and ignition circuits in general, have been proposed in the past. One of the problems encountered in spark ignition circuits is spark noise and, generally, heretofore accepted ignition circuits have been objectionable on the basis of generating excessive noise. A further shortcoming of such prior circuits is that they have generally not been readily adaptable to functioning in cooperation with control circuits. This invention is concerned with the problem of spark noise and also in providing a circuit which is particularly adaptable to use in connection with various control circuits.

SUMMARY OF INVENTION In accordance with this invention an ignition circuit is provided wherein an ignition transformer is connected to a pulsing circuit which includes a capacitor in series with the transformer primary and the electrical source lUnited States Patent O 3,463,963 Patented Aug. 26, 1969 ICC supplying electrical power to the ignition circuit, arcing electrodes being connected in the transformer secondary. A switch arrangement is provided in circuit with the capacitor and the transformer primary and is operative to alternatively interrupt and momentarily complete the circuit of the capacitor and the transformer primary so that a steep wave front pulse is applied to the transformer primary. In accordance with a more specific aspect of this invention the switch arrangement is itself electrically responsive so that the occurrence of the momentary pulse is controllable. In one alternative a voltage divider circuit is provided to control the electrical state of the switch arrangement. The pulsing circuit can be controlled on the basis of a sensed condition. In one specific variation a thermistor is included in one branch of the voltage divider circuit whereas in another variation one branch of the voltage divider circuit includes the primary of a voltage step-down transformer with the secondary including a low voltage thermostat arrangement. Both variations are capable of controlling the occurrence of the ignition pulse-on the basis of a sensed temperature condition.

DESCRIPTION OF DRAWINGS FIG. 1 is a circuit diagram of an embodiment of this invention;

FIG. 2 is a partial circuit diagram of an alternative embodiment; and

FIG. 3 is a View of the pulsing transformer.

DESCRIPTION OF PREFERRED EMBODIMENT The ignition circuit of FIG. l is illustrated as including spaced electrodes 10 and 12 connected to the secondary 14 of transformer 16. Electrodes 10 and 12 are associated with a suitable fuel burner (not shown) so that a spark drawn between the electrodes will ignite fuel being emitted from the burner. The ignition transformer is connected to a pulsing circuit and a source 18 of alternating current. The pulsing circuit includes silicon control rectifier 20 and capacitor 22 connected in series with each other and primary coil 24 of the transformer.

As mentioned previously, one of the problems encountered in spark ignition circuits is lspark noise and this invention is concerned with the problem of reducing spark noise as well as with providing a simplified and versatile ignition circuit. In operation and assuming the SCR to be non-conductive, the SCR will become conductive when the necessary potential appears at its gate 32. This potential is proportional to the source voltage and is determined by the relative values of resistances 26 and 28 and the value of capacitor Z2. Resistance 26 is connected across the anode 30 and gate 32 of the SCR whereas resistance 28 is connected across the gate 32 and the cathode 34 ofthe SCR. The values of these elements (resistances 26 and 28 and capacitor 22) are chosen to that the triggering or switchover voltage of the SCR is reached when the source voltage reaches a value close to its peak voltage on each positive half cycle. At the switchover voltage the SCR is rendered conductive and current occurs in primary 24 ofthe transformer as the capacitor charges. The capacitor, being in series with the primary, can be of a relatively small size so that it will charge and discharge rapidly. This results in a pulse in the primary coil having a steep wave front, i.e. a fast rising, short duration pulse. With the steep wave front, the frequencies of the spark components are relatively high and the steeper the Wave front the higher will be the frequencies. The spark drawn across the electrodes as a result of this pulse is a high voltage, low energy spark and with those characteristics the noise normally attendant the spark is substantially reduced. It is believed that the reduction in noise is due, in large part, to the fast rising, -short duration pulse which is not sustained across the electrodes for any undue length of time. As previously mentioned, the series connection of the transformer primary and the capacitor permits the use of a relatively small capacitor which is then relatively rapidly charged to produce the steep wave front pulse. lBy Way of example, and assuming a 115 volt AC source, capacitor 22 can have a value of .33 mf., with these values the current pulse in the primary has typically a rise time of three microseconds and a peak of eight amperes. Also, resistance 28 is preferably a thermistor to compensate for changes in gate switchover, or triggering, voltage due to variations in temperature of the SCR ambient. In the illustrated embodiment thermistor 28 has a negative temperature coeiiicient of resistance.

This arrangement whereby a fast rising, short duration pulse is periodically applied to the ignition transformer results in a substantial reduction to spark noise. In this connection transformer 16 is also provided with a specilic construction which experience has shown also contributes to a reduction in spark noise. With reference to FIG. 3, the primary 24 of the transformer is wound on a core 36 in any conventional manner and the secondary coil 14 is also wound on the core so that it, along with the primary, is inductively related with the core. In winding the secondary coil a irst coil section 14a is wound radially outward from the core but in a limited axial area with respect to the core. When the desired radial depth of coil is reached, an annular insulating spacer 38 is placed against an axial end of the wound coil section 14a. The wire is taken over the spacer and winding continued to provide a second coil section 14b wound radially outward on the core and again in a limited axial area. When the second section has reached the desired radial depth, another spacer 38 is placed on the exposed axial face of the coil section. This winding procedure is repeated until the entire secondary is Wound in discrete sections one insulated from the other by spacers 38. It has been observed that this type of transformer results in a reduction in spark noise as compared to other, conventionally wound transformers, for example transformers wound in a continuous axial and radial manner on the core and not in discrete axial sections as this transformer. An insulating tape, or other suitable covering material, is placed over the wound secondary.

A further desirable characteristic resulting from this transformer construction is a reduction in insulation problems as compared to other transformer designs. For example, in the above noted conventionally wound transfor-mer where the coil is wound in axial and radial layers continuous over the transformer core, it is generally assumed that the last coil turn will be spaced from the first turn by the entire core length but there is no assurance that this condition will be obtained. The possibility exists that the last turn may, due to loose winding, be displaced radially inward and axially toward the first turn from the position it should assume, thus the last turn will be closer to the first turn than expected. In a pulsing ignition transformer the secondary coil may be subjected to voltages in the range of 10,000 volts. If the end turns are too close together then they must be adequately insulated to prevent arcng with a potential dilference of 10,000 volts between the end turns. In the same situation, i.e. under the same voltage conditions, the maximum voltage drop which must be considered with the transformer construction of this application is that between adjacent turns in adjacent axial sections 14a and 14b, and the maximum voltage drop in that area would be for less than 10,000 volts depending on the number of axial sections. With twelve such sections as shown the maximum voltage drop is approximately 900 volts.

The basic pulsing circuit wherein the provision of the momentary pulse in the primary is controlled by a switch arrangement which is itself electrically responsive renders the circuit particularly adaptable to a control application wherein ignition is produced on the basis of an external ,4 condition. It was previously pointed out that the relative values of resistances 26 and 28 are a factor in determining the switchover or triggering point for the SCR. As is well known in SCR technology, the SCR will conduct when its anode and gate are positive with respect to its cathode and when the gate reaches a particular potential with respect to the cathode. This difference in potential is then determined by the relative values of resistances 26 and 28. In the basic circuit the gate voltage increases during each positive half cycle until the SCR is eventually triggered on. FIG. l illustrates a form of circuit connection wherein a control function is achieved and ignition is controlled on the basis of an external condition.

In FIG. l thermistor 40 is connected in parallel with resistance 28 and cooperates in determining the potential at` point 42 which then determines the gate potential. Thermistor 40 can be exposed to a particular temperature condition and will determine the triggering point for the SCR on the basis of that temperature condition. To provide additional control over the switchover point, variable resistance 44 is also connected in parallel with thermistors 40 and 28. This arrangement provides a voltage divider circuit connected with the thermistor such that the potentials at circuit points 42 and 46, which correspond to the gate and anode of the SCR, are determined by the characteristics of the voltage divider circuit. Thermistor 40 has a negative temperature coetiicient of resistance and provides variable characteristics in the voltage divider circuit, one branch of which consists of thermistor 40, either alone or with resistances 28 and 44. The resistance of the voltage divider branch including thermistor 40 will vary in accordance with the temperature to which the thermistor is exposed so that, for example, when thermistor 40 is above a predetermined temperature the SCR will be non-conductive but below that temperature it will be rendered conductive. More particularly, as the temperature of thermistor 40 increases, its resistance value decreases. Thus, when its temperature is relatively high the potential at point 42, and on the SCR gate 32, is low with respect to the Volltage needed to render the SCR conductive and the SCR will be in a non-conductive state. When the temperature of the thermistor' 40 falls below a predetermined value its resistance value increases to a point that the potential at circuit point 42 and 0n the gate is suciently high with respect to the cathode to switch the SCR on and produce an ignition pulse in the transformer. The thermistor 40 is exposed to the temperature condition on the basis of which the control is to be achieved. In the case of an oil or gas burner it can be exposed to the area being heated by the burner. The control can also be used in other control environments such as to control a clothes dryer or the like by exposing the thermistor to temperature of the exhaust air of the clothes dryer. Resistance 44 can be a'djusted to vary the temperature at which the SCR will be rendered conductive. In the sense that the SCR is either conductive or non-conductive it can be looked upon as a switch 'and in the sense that it is switched on in accordance with a temperature condition, it and its associated circuitry can be looked upon as a thermally responsive switch.

At this point it should be understood that the SCR, shortly after being turned om will be turned off during each positive half cycle but will be turned on again during the next positive half cycle if the requisite voltage condition is present at the SCR gate. Conduction through the SCR will occur only on each positive half cycle and then only if the temperature sensed by the thermistor indicates a need for increased head.

FIG. 2 illustrates still further the versatility of the basic pulsing circuit, namely it is adaptable to use with a low voltage thermostat. Elements corresponding to those included in the circuit of FIG. l are identified by the same reference numeral in FIG. 2 as in FIG. l. In

this variation thermistor 40 and variable resistance 44 have been replaced by a voltage divider circuit including a transformer 46 and a resistance 48. The juncture 50 between the transformer and resistance 48 is connected to the SCR gate 32 through resistance S2. Primary 54 of transformer 46 is in one branch of the voltage divider circuit and resistance 48 is in the other branch and both are connected across source 18. A thermostat is connected with secondary coil 56 of the transformer, the contacts 58 of the thermostat are normally open and are in series with the secondary coil. Transformer 46 is a voltage step-down transformer so that a low voltage condition obtains in the transformer secondary and at the thermostat. Assuming the 115 volt AC source, the transformer 46 is such that six volts is` induced in the secondary. The thermostat will function with the six volt condition and this provides a low voltage control and permits relatively small leads to be used in the control thereby rendering the control adaptable to most heating applications. When contacts 58 of the thermostat are open the impedance of primary coil 54 is relatively large with respect to that of resistance 48. Thus a relatively greater voltage drop occurs across primary 54 than across resistance 48 so that the potential at juncture S0, and on the SCR gate, is relatively low or at least below the triggering voltage of the SCR. When the thermostat senses a condition requiring heat, its switch contacts are closed thereby shorting the transformer secondary and resulting in relatively high current in the transformer. This high cuirent condition is translated back to the transformer primary through conventional transformer action and reduces the impedance of the primary coil so that the voltage divider circuit now has a relatively higher potential across resistance 48 than across coil 54. This places a higher potential on juncture 50 and establishes the condition for triggering the SCR on. The remainder of the pulsing circuit is identical to that discussed in connection with FIG. 1 with the advantage in this alternative being `that it permits control of the ignition circuit through a relatively low voltage arrangement.

Up to this point, attention has been directed only to the ignition portion of the circuit. The overall circuit illustrated in FIG. l is also adapted to control fuel delivery to the burner which is associated with the spark electrodes. Fuel control is achieved on the basis of the success or failure of ignition and using the voltage in the ignition transformer secondary as the control parameter. This portion of the circuit operates on the same basic principle as that disclosed in Patent 3,291,183 of Richard K. Fairley and assigned to the assignee of this application. Similar to the disclosure of that patent, this circuit will maintain fuel flow to the burner only if ignition is successful and will automatically terminate fuel flow if ignition fails to occur. The illustrated fuel ow control circuit is more specifically disclosed in the copending application of Richard K. Fair-ley and Reno L. Vicen'zi, Ser. No. 694,201, led Dec. 28, 1967 and entitled Ignition and Fuel Control Circuit and assigned tothe assignee of this application. The illustrated circuit will be described in a general manner to complete the disclosure of this application but, should a more specific description of the circuit become necessary, reliance is hereby placed on the aforementioned copending application.

In the circuit transformer 16 is provided with a tertiary winding 60. Winding 60 is structurally Wound over the tape cover and the exterior of secondary winding 14, see FIG. 3, so that it senses the secondary voltage with a minimum of interference from the primary. The voltage induced in winding 60 is proportional to the secondary voltage and this voltage condition is used to control the fuel flow control valve.

The fuel flow control valve is a solenoid actuated valve and the solenoid 62 of the valve is illustrated as connected in series with transistor 64, .e. in series with the emitter and collector of the transistor. Coil 60 is connected to the transistor base through diode 66 and capacitor 68. Source voltage is applied to the transistor through diode 70, diode 70 and capacitor 74 providing a rectified supply for the transistor. The voltage induced in coil 60 is applied to base 76 of the transistor which then determines the magnitude of current in the emittercollector circuit of the transistor and correspondingly, in solenoid 62. The voltage in coil 60 is proportional lto the voltage in coil 14 which in turn is dependent on the condition at the electrodes 10 and 12. Thus the current in the transistor is dependent on the condition at the electrodes 10 and 12.

More specifically, the voltage condition at the electrodes can be either high when ignition pulses occur across the electrodes with no llame present, medium when ignition pulses occur across the electrodes with a ame present, low when ignition pulses occur across shorted electrodes, and zero when pulses do not occur. As discussed more completely in Patent 3,291,183, this fuel control circuit takes advantage of the fact that due to ionization of the spark gap which occurs after ignition the potential necessary to arc across the gap with a flame present is substantially less than with no flame present. This change in condition can be utilized in the control of a solenoid operated valve where the valve requires relatively high voltage or current for initial opening but will be maintained open with reduced voltage on or current in the solenoid of the valve.

Returning now to the specific circuit illustrated, assuming no flame present at the electrodes and ignition pulses supplied to the transformer, a relatively high voltage will appear across the electrodes and will be translated into a relatively high voltage in coil 60. This 4will be translated into relatively high current in the emittercollector circuit of the transistor. Resistance 78 is selected so that under this high voltage condition the amount of current in the emitter-collector circuit is suicient to permit solenoid 62 to open its associated valve. This opens the fuel flow valve and supplies fuel to the burner so that ignition can occur. Upon Successful ignition a flame is present between electrodes 1'0 and 12 thereby reducing the voltage condition on the electrodes. This reduced voltage condition is translated through coil 60 to reduced current in the emitter-collector circuit of the transistor but the reduced current is sufficient to hold the fuel valve open. Thus the fuel ow control circuit automatically adjusts to successful ignition and maintains a continued supply of fuel to the burner.

The circuit is also adapted to automatically close the fuel flow valve in the event that ignition does not occur or if for some reason the burner should -be extinguished and reignition not occur. To this end, a circuit breaker consisting of heater 80 and switch contacts 82 is pro vided in series with solenoid 62. The circuit breaker is such that it will not operate in response to the reduced current condition which exists in solenoid 62 in the event of successful ignition or less current; however, should a relatively high current condition persist indicating a failure of ignition at electrodes 10 and 12 the circuit breaker will respond by opening contacts 82 and allowing the valve to close thereby automatically terminating supply of fuel to the burner. The circuit breaker is selected so that its time constant (the time required before opening the circuit) is sufficiently long to permit ignition to occur under normal conditions.

It will also be noted that the fuel flow valve will only open and supply fuel to the electrodes if the electrodes are being pulsed to produce ignition. Similarly, if the ignition pulse is present but the electrodes are shorted a relatively low voltage condition exists in the ignition secondary and this is translated into relatively low current in the emitter-collector circuit of the transistor which is insufficient to open valve 62 or hold the valve open if already open when the electrodes become shorted.

Although this invention has been illustrated and described in connection with particular embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

What is claimed is:

1. An ignition circuit comprising, in combination,

an electrical source,

a transformer,

arcing electrodes connected in the secondary of the transformer,

and circuit means for connecting the primary of the transformer to said electrical source, said circuit means including capacitance means in series with said transformer primary and said source, and switch means in circuit with said capacitance means and said transformer primary and operative to interrupt the circuit of said capacitance means and said transformer primary and `further operative to momentarily establish the circuit of said capacitance means and said transformer primary so that a steep wave front pulse is applied to said transformer primary and a high voltage, low energy content spark occurs across said electrodes.

2. The ignition circuit of claim 1 wherein said switch means has a non-conducting state in which it interrupts the circuit of said capacitance means and said transformer primary and is switched to a conducting state in response to a predetermined voltage condition on said switch means,

and including circuit means connected in circuit with said switch means and cooperating in determining the voltage condition on said switch means.

3. The ignition circuit of claim Z' wherein said circuit means comprises a voltage divider circuit in parallel with said switch means with respect to said source and including temperature responsive thermistor means adapted to be exposed to a ternperature condition so that the voltage condition on said Lswitch means corresponds to said temperature condition and the occurrence of said pulse is dependent upon said temperature condition.

4. The ignition circuit of claim 2 wherein said circuit means comprises a voltage divider circuit in parallel with said switch means with respect to said source and including a voltage step-down transformer the primary of which is connected in said voltage divider circuit,

and thermostat means connected in circuit with the secondary of said step-down transformer and adapted to be exposed to a temperature condition and operative to close the secondary circuit at a predetermined temperature and thereby alter the characteristics of said voltage divider circuit so that the occurrence of said pulse is dependent upon said temperature condition.

S. The ignition circuit of claim 3 wherein said switch means comprises a silicon controlled rectifier having an anode, a cathode and a gate,

and said voltage divider circuit is connected to said silicon controlled rectifier with said thermistor means determining the voltage condition of said gate.

6. The ignition circuit of claim 5 wherein said voltage divider circuit provides first and second branches, said first branch connected across the anode and gate of said silicon controlled rectifier and said second branch across the cathode and gate of said silicon controlled rectifier,

and one of said branches has a relatively fixed resistance value and the other branch includes said thermistor means.

7. The ignition circuit of claim 6 wherein said ignition transformer comprises transformer comprises a core,

a primary coil inductively related with said core,

and a secondary coil inductively related with said core and comprising a plurality of adjacent coil sections arranged serially with an insulating medium between each section and each section electrically connected to an adjacent section.

9. The ignition circuit of claim 8 wherein said core is elongated,

said coil sections are serially arranged in an axial direction on said core,

and said insulating medium comprises a generally annular member disposed between adjacent axial faces of said coil sections.

10. The ignition circuit of claim 4 wherein said switch means comprises a silicon controlled rectifier havingan anode, a cathode and a gate,

and said voltage divider circuit is connected to said silicon controlled rectifier with the voltage condition at said gate being dependent upon the operative state of said step-down transformer.

11. The ignition circuit of claim 10 wherein said voltage divider circuit includes first and second branches with the junction of said first and second branches connected to said gate, one of said branches having a relatively fixed value resistance and the other branch including the primary of said step-down transformer.

12. The ignition circuit of claim 11 wherein said ignition transformer comprises a core,

a primary coil inductively related with said core,

and a secondary coil inductively related with said core and comprising a plurality of adjacent coil sections arranged serially with an insulating medium between each section and each section electrically connected to an adjacent section.

13. The ignition circuit of claim 5 wherein said source an AC source.

14. The ignition circuit of claim 10 wherein said source an AC source.

15. An ignition circuit comprising, in combination,

an electrical source,

a transformer,

ignition means connected in the secondary of said transformer,

switch means in circuit with the primary of said transformer and said electrical source and operative to interrupt the circuit to said transformer and further operative in response to a predetermined electrical condition, to momentarily establish the circuit to said transformer primary so that a pulse is applied to said primary,

and temperature responsive means connected to said switch means and adapted to be exposed to a temperature condition, said temperature responsive means operative in response to a predetermined temperature to impress said predetermined electrical condition on said switch means.

16. The ignition circuit of claim 15 wherein said temperature responsive means comprises thermistor means.

17. The ignition circuit of claim 16 wherein said switch means comprises a silicon controlled rectitier,

and said thermistor means is connected to the gate of said silicon controlled rectiier and controls the operative state of said silicon controlled rectifier.

18. The ignition circuit of claim 15 including means dening a voltage divider circuit connected to said switch means,

one branch of said voltage divider circuit including the primary of a voltage step-down transformer,

thermostat means connected in the secondary of said voltage step-down transformer,

and wherein the operative state of said switch means is dependent upon the operative state of said thermostat means and said voltage step-down transformer.

19. The ignition circuit of claim 18 wherein isaid switch means comprises a silicon controlled rectifier,

3,045,148 7/1962 McNulty et al. 315--183 3,282,324 11/ 1966 Romanelly `43 l-24 3,311,789 3/1967 Remy 317--96 X 3,384,439 5/1968 Walbrdge 431-24 3,3 84,440 5/ 1968 Mayer 431-66 VOLODYMYR Y. MAYEWSKY, Primary Examiner U.S. C1. X.R. 

